This invention relates to fluid blast circuit breakers including the puffer type and the two-pressure storage type. More particularly, this invention relates to a novel contact and nozzle configuration which enhances arc cooling by the fluid blast to facilitate extinguishing the arc created by opening the contacts.
When contacts are opened in a fluid blast circuit breaker, an arc is created between the contacts. The arc is formed of a plasma of ionized gas which carries the arcing current. When the heated plasma is cooled, its electrical resistivity increases markedly until the plasma is no longer able to carry the arcing current. In an alternating current system, the energy content of the arc at zero crossing of the current is low. If an adequate supply of cooling fluid is applied to the arc near the instant of zero crossing, the plasma is cooled adequately to interrupt the arc and thereby break the circuit. It is known in the circuit breaker art that the flow component of the extinguishing fluid in the direction perpendicular to the arc restricts the diameter of the arc, which facilitates cooling of the arc improving the probability of interrupting the arc.
The decisive physical characteristic which controls the restriction of the cross sectional dimensions of the arc is the velocity of the fluid flow perpendicular to the arc. The prior art techniques have attempted to improve arc-extinguishing fluid flow by bringing nozzles or nozzle structures into the arc path. All of the known prior art structures used to increase the radial velocity of the fluid flow have been of circular cross section. For example, in U.S. Pat. No. 3,551,624 issued Dec. 29, 1970 to Fischer, additional structure is introduced into the arc path to eliminate high back pressure exerted by the arc on the fluid flow, and, in U.S. Pat. No. 3,946,180, issued Mar. 23, 1976 to Votta et al., additional parts are provided in the nozzle to facilitate fluid flow into the arcing region. The limitation on the diameter of the nozzle is that its diameter must be adequate to avoid choking, i.e., blocking the flow of fluid over the arc by providing inadequate space between the nozzle and the contact structure. A similar choking problem exists in attempting to increase fluid flow velocity in structures having square nozzles.
Therefore, the primary object of the instant invention is to provide a nozzle structure which allows increased radial velocity of the arc extinguishing fluid without choking the fluid flow.